Helium isotopes promise geothermal energy

In a tour of the northern western US intertidal basin, the geochemist Mack Kennedy of the Department of Energy, the Arizona Lawrence National Laboratory and Matthijs Van Soest National University, discovered a tool. help identify potential geothermal energy sources.

Currently, most geothermal energy is exploited from volcanic activity areas such as geysers of northern California.The hidden energy discovered by Kennedy and Van Soest does not originate from volcanoes but from fluid flows through deep fissures to the inner crust of the earth in areas far away from where they were or are there are active volcanoes. They announced their search results in Science on November 30, 2007.

At the Berkeley lab, B. Kennedy's geochemist used this great spectrometer (close-up) to determine the ratio of helium isotopes in the surface sample taken from the northern mountain basin. .(Photo: Roy Kaltschmidt)

'A good source of geothermal energy must ensure three basic requirements: high heat gradient - meaning that the stone must be hot enough to use it - plus the supply of self-replicating fluids (usually water) and finally "The fluid can flow through hot rocks through penetrating deep roads," said Kennedy, a scientist at Berkeley Laboratory's Earth Science unit. 'We have found a way to map and determine the number of osmotic areas deep within the earth's crust that are generated by tectonic activity - the movement of the earth's crust - not volcanic activity. . "

Kennedy and Van Soest compared the ratios of helium isotopes found in samples taken from wells, streams and water courses throughout the northern basin. Helium-3 isotope - the nucleus has a neutron only in the stars, the earth's crust retains a large amount of the original helium isotope (compared to the very small amount in the air) remaining after the formation of the solar system. On the other hand, the earth's crust is rich with radioactive elements such as Uranium and thorium, which are broken down by shining alpha rays and becoming the nucleus of Helium-4 isotopes. Therefore, a high proportion of Heli-3 and Heli-4 in fluid samples shows that most of this fluid is from the earth's crust.

The volcanic regions all have high Helium ratios, where fluids penetrate through the thin boundaries of the inner crust. But when Kennedy and Van Soest found that high Helium regions were located far away from the volcanic region, they understood that fluids certainly penetrated this delicate boundary in another way.

The geology of the area is the focal point. The basin with mountains is characterized by mountains stretching from north to south separated by wide and relatively flat valleys. Here, each block of rock and soil is subsided by layers of eroded sediments from the raised mountain range. The terrain of the basin alternates mountains alternating with the process of expanding the earth's crust from east to west about 30 million years ago. This is where the world's thinnest earth crust is, so it has caused unusually high gradient heat phenomena.

Geochemist Matthijs van Soest is taking surface water samples in the intertidal basin (Photo: Mack Kennedy)

The surface of the mountain range in the intertidal basin clearly shows the normal milling, ie the mountains are separated from each other due to the expansion of the earth's crust. These mills form steep paths deep inside a fragile outer shell. But when the main axis of milling touches the inner soft shell, changing the density and ductility of the stone refracts the law of the force acting on the milling, making the milling axis more horizontal. Because of these deep and horizontal tendencies, Kennedy thought that osmosis pathways could penetrate from the delicate boundary into the earth's crust.

One of the areas with strong seismic activity in the basin is called Nevada central seismic belt. Their detailed studies in this area are primarily the Dixie Valley heat system located next to the Stillwater Range, which proves that the highest ratio of helium in fluids comes from the face milling system in front of the Stillwater Range. .

The northern intertidal basin, which Kennedy and Van Soest studied, represents the DOE agency on Basic Energy Science and Geothermal Technology that stretches across five states: California, Nevada, Oregon, Idaho and Utah. In the study, they outlined steady progress from low helium ratios in the east to high proportions in the west. The rate of increase is similar to the rate of increase and the change in the expansion direction of the earth's crust, from the east-west through the basin alternating mountains to the northwest.

The change in speed and direction reflects the additional shear strain due to the movement of northward movement of the Pacific shelf across North America. Kenneday and Van Soest believe that the added component of sliding deformation and the expansion rate are opening up the way for fluid to pass through the thin inner crust to the earth's outer shell. High ratios of helium isotopes have been found to promise latent geothermal energy plus a general context: areas with high rates often have permeability above average.

'We have never witnessed such a clear connection between the geochemical sign of the earth's surface with tectonic activities, as well as never being able to determine the depth of penetration from any method. any surface, ' said Kennedy. The specimens obtained from the surface have paved the way for them to study the structure of the rock layer deep below without drilling.

When the search for renewable energy sources that do not produce greenhouse gases is an urgent problem, geothermal is an ideal source of energy - 'the best self-renewable energy source after the sun' , Mr. Kennedy said. The source of geothermal energy being exploited in the United States, except Alaska and Hawaii, has produced about 90 million million kilowatts per hour, more than 3000 times the country's annual energy consumption. Determining helium ratio by surface method is a practical way to locate a promising new energy source.